Method and apparatus for minimizing self-excited oscillations in vacuum-tube signaling devices



Feb. 22, 1927. I 1,618,319

J. WEINBERGER METHOD AND APPARATUS FOR MINIMIZING SELF EXCITED OSCILLATIONS IN VACUUM TUBE SIGNALING DEVICES Filed Feb. 7. 1921 2 Sh'eets-Sheet 1 v INVENTOR JUL/m 1427110957265)? ATTORNEY METHOD AND APPAR-ATUS ,OSCILLATIONS IN VAC WEI Lfiwm NBERGER I FOR MINIMIZING SELF EXCITED UUM TUBE SIGNALING DEVICES Filed Feb. 7, 1921 2 Sheets-Sheet 2 V 71 If g 7, 6 f 1 W 77 L W a X & g

W/ f fi 5 INVENTOR JVZ/[w WEI/K3511 651? Um MW ATTORNEY Patented Feb. 22, 1927.

STATES JULIUSWEINBERGER, OF NEW YORK, N. Y. ASSIGNOB TO RADIO CORPORATION OF I AMERICA, A CORPORATION OF DELAWARE.

METHOD AND API ARATUS FOR MINIMIZING SELF- EXCITED OSCILLATIONS IN VACUUM-TUBE SIGNALING DEVICES.

Application filed February 7, 1921. Serial No. 443,318.

is especially useful in obtaining extremely high amplification of audio or radio free quency electromotive forces.

-There are two main forms or types of amplifier circuits, known in the art respectivelyas transformer coupled, and resistance coupled, but a great deal of trouble has been experienced in the operation of such amplifiers, due to self-excited oscillations generated with. the amplifyin circuits. These oscillations give rise to a owling tone, "or to irregular noises in the telephone receiver, -which sometimes makes it impossible for the operator to hear anything e se.

I have found that this trouble is due in large measure to capacity between various parts of the amplifier, the most troublesome capacity generally'being that between the telephone leads in the final output circuit or the observer, and the input circuits of the am lifier.

' nother source of trouble is found in the capacity between various elements of the amplifier circuits themselves, as the con-- 'denser's, vacuum tubes, or resistances in a resistance coupled amplifier; oscillations occuring between intermediate vacuum tubes in the;amplifier. These oscillations are of extremely high frequency, the elements of between vacuum tubes and the lead wires.

However, even though the oscillations themselves may be of inaudible frequency, they are usually broken up by the intermittent charging and leaking of the grid condensers, and so produce a tone or noise in the telephones.

the oscillating circuits being the condensers The primary object of the present invention 1s minimize or reduce the possibility of smgmg or self-excited oscillations by electrostatically shielding the input and output circuits and. the apparatus contained therein, and also by electrostatically shielding each element withinthe amplifier from every other element by appropriate means. To theseends; asuitable apparatus and tubes for carrying out my method are illus trated in the accompanying drawings, in which, Fig. 1 is a ,dia rammatic representation of atypical tr'an'stormer coupled amplifier, of the ordinary type; i Fig. 2 is a similar view of a typical resistance coupled amplifier. These views in Figs. 1 and 2 are for the purpose of illustrating the association of elements which require shielding in accordance with my invention;

Fig. 3 is a diagrammatic representation of a capacitive coupling existing between output and input circuits in a typical amplifier circuit,

Fig. 4 is a of a transformer coupled-ampli er likethat shown in Fig. 1 with my invention applied thereto; and V a Fig. 5 illustrates a resistance coupled amplifier like that shown in Fig. with my invention applied thereto.

Referring to the drawings, and at first more especially to Fig. 1, a suitable source of energy to be employed (not shown may be connected to the terminals A an B of an input transformer C, the-secondary of which is connected to the grid D and filament E (input terminals) of a vacuum tube F of any suitable type, in this instance a tube of the three element type being shown, having the anode or cold plate G. Two tubes F are indicated in this instance, although the principles of the invention may be applied in any desired number of types.

The filaments E of the tubes F are energized from a common battery H and the necessary plate potential 'for' "operating all the tubes is provided by the battery J. The output terminals of the; first tube F, that is the plate G and the positive end of diagrammatic re resentation battery J, are connected to the primary of a second transformer K, the secondary of which is connected to the grid D and filament E of the second vacuum tube F. The output terminals of this second tube F are connected to an electro-responsive device, as a pair of head telephones L, in which the energy is to be absorbed.

In the resistance coupled type of amplifier shown in Fig. 2, the source of energy to be amplified (not shown), may be conne'cted to the terminals and P and is thus impressed across the input side, that is across the grid D and filament E of the amplifying tube F, a condenser Q, being connected in this instance in the lead 0 as shown. The output terminals of the second tube F are connected as shown to an energy absorbing device which may be the head telephones L.

The troublesome capacity generally existing between the telephone leads, or the observer, in the final output circuit, and the input circuits of an amplifier, may be diagrammatically represented as in Fig. 3. In this figure, the vacuum tube F may be considered as representing the numerous types of amplifier, it being assumed for purposes of illustration that the amplification ofthis single tube F is substantially equal to that of all the tubes in an amplifier. That is, the

entire aggregate of input circuit, amplifying tubes and output circuit, may be reduced to a single tube of equal amplification and with the same or equivalent input and output circuits connected to it. The input circuits may be represented in Fig. 3 by the inductance L, resistance R, and capacity C, and the output circuit may be represented by the inductance L resistance R and capacity C The capacitive coupling existing between input and outputcircuits may be represented by the capacity C. Theoretical consideration of such a system will indicate that for given values of the input and output circuit constants, the higher the amplification of the tube F, the less will be the value of the capacity C required to roduce selfexcited oscillations. For big 1 amplifications this maybe incredibly small. For example, with one winding of a transformer of the ordinary telephone repeater coil type connected to the input termmals of an amplifier having an overall voltage amplification of about one hundred times, and with a pair of ordinaryrradio receiving telephones in the output circuit, the capacity between the observer wearing the telephones and the input transformer windings, even with the observer several feet away, is sufiicient to set up strong singing. With voltage amplifications of the order of five thousand times, self-excited oscillations are set up easily, regardless of the constants of the input and output circuits, unless means are providedfor preventing or reducing the production of self-excited oscillations in the amplify: ing circuits.

My method of solving the problem deals largely with the elimination of capacity between various points in the circuits. As stated, the most troublesome capacity -generally exists between the telephones in the output circuit and the observer, and the apparatus of the input circuit.

As shown in Fig. 4, I overcome this difficulty by surrounding the telephone cords with a shield preferably of copper braid T, as shown in F igs. at and 5, and by connecting the metal caps and metal head band U of the telephones L to the shield T around the cords, and in general taking every precaution to obtain a complete shield around all parts of the output circuit. This shield may then be connected to earth, or to a filament lighting battery H of the amplifier. In this way, the capacity between final output and input circuits is not only reduced substantially to zero, but there is interposed between the observer and the output circuit a grounded shield, so that the observer is no longer capacitively connected to the output circuit via the telephones, and thus .any capacity between the observer and input circuit is rendered substantially harmless.

The oscillations between various elements" of the amplifier circuits themselves, as for lnstance the condensers, resistances, and

vacuum tubes, may be eliminated by. plac-i ing each amplifying element in a metal box V, preferably of copper and suitably grounded, An amplifying element may consist of a vacuum tube F and the condenser and resistances with which it is immediately associated as illustrated in Figs. 4 and 5. By means of the metal boxes or metal shields V, a multi-step amplifier is produced in which each vacuum tube F with its immediate plate and grid circuit elements is shielded from the other vacuum tubes with their plate and grid elements, and mutual capacity between the various steps of the amplifier reduced substantially to zero.

The shielding between vacuum tubes is not always necessary, provided the required amplification is fairly low, In this case, however, it is desirable to enclose the vacuum tubes and their associated amplifier circuit elements in a metal box ,V so as to reduce mutual capacity between the vacuum tube circuits and the input circuit to substantially zero. The shielded telephones L may then be brought out of the amplifier case or box V as indicated in Figs. 4 and 5. Capacitive coupling between input circuit, the elements of the amplifier and the output circuit is thus minimized or eifectually prevented.

In case shielding of the output circuit is diflicult, that is, in case the output of the amplifier is to be impressed on a piece of apparatus which is larger than can be conven-- iently shielded, substantially the same results may be accom lished by shielding the transformers C and li in Fig. 4. As shown, a metallic grounded shield W is inserted between the primary and secondary windings,

'so that no capacitive coupling exists beneed not be shielded since these circuits are ordinarily grounded in any event, but I have found it generally advisable to provide electrostatic shields for all the other circuits and lead wires of the amplifier, and as shown in Figs. 4 and 5 the said circuits and lead wires are covered with'the shields of copper braid T as indicated for the telephone cords.

I claim and desire to obtain by Letters Patent the following:

1. In a telephone receiver circuit the combination of telephone cords, an amplification circuit andmeans for shielding said cords from extraneous electrical influences comprising a metallic shield enclosing said cords substantially throughout their length and an electric connect-ion between'said shield and said amplification circuit.

2. The combination of telephone cords, .a

low potential circuit, means for shielding the cords from extraneous electrical influences, said means including a metallic shield, an electrical connection between the shield and the shell of the telephone receiver and an electrical connection betweenthe shield and low potential circuit.

3. In a system for amplifying feeble electricalcurrents, the combination with a signal amplifying device, a low potential circuit in said signal amplifying device, a signal responsive device, conductors connect-- ing said devices, a shield surrounding said conductors substantially throughout their length, and a connection between said shield and said low potential circuit.

4. A high power radio receiving amplifier comprising an electron tube' having grid, filament and plate electrodes, a telephone receiver actuated by said amplifier, and means for shielding the telephone receiver from extraneous electrical influences and reducing regenerative efi'ects, said means including a metallic shield surrounding the telephone having a low 'vacuum tubes having conductors substantially throughout their ,length and a connection between said shield and one of said electrodes.

5. A high power radio receiving amplifier. a telephone receiver associated therewith and means, for shielding the telephonereceiver from extraneous electrical. influences,

said means including a metallic shield sur rounding the telephone conductors, and an electrical connection between theshield and the tele hone receiver.

6. A igh power radio receiving amplifier otential circuit, a telephone receiver associated therewith, conductors connecting said receiver with said amplifier and means for shielding the telephone receiver from extraneous electrical influences, sald means including-a metalllc shield surrounding the telephone conductors and an electrical connection between-the shield and the low potential circuit.

7. A high power radio receiving amplifier having a low potential circuit, a telephone head set associated therewith, and means for shieldin the telephone head set ill) from extraneouse ectrical'influences, said means including a metallic shield surrounding the telephone conductors, and electrical connection between the shield andthe telephone head set, and an electrical connection between' the shield and. the low potential circuit. v

8. -In a radio receiving apparatus the combination of a thermionic vacuum tube amplifier, an input circuit, an output circuit, reproducing means in said output circuit and means surrounding the conductors of said output circuit and extending substantially throughout the length thereof for shielding said reproducing meansfrom extraneous electrical influences and regenerative-effects.

9. In a radio receiving apparatus the combination of a plurality of thermionic vacuum tubes having grid, filament and plate electrodes, circuits interconnecting said -electrodes, an. output circuit including a plate electrode, telephone receivers and a source of potential and means surrounding the conductors of said circuit and extending substantially throughout the length thereof "for shielc .ng said telephone receivers from extraneous electrical influences and regenerative effects.

10. In a radio receiving apparatus the combination of a plurality of thermionic id, filament and plate electrodes, circuits mterconliecting said electrodes, ,asource of potential saidfilament electrodes, an output circuit including a plate el trode, telephone receivers and a source 0 potential and means connected to said source of filament otential surrounding the conductors of sai circuit substantially throughout their length for shielding said telep one receivers from ex- Inn or heating traneous electrical influences and regenerat1ve effects.

11.111 a radio receiving apparatus the tential surrounding the conductors of said output circuit and extending substantially throughout the length thereof for shielding said reproducing means from extraneous electrical influencesand regenerative effects.

12. In a radio receiving apparatus the combination of a plurality of thermionic vacuum tubes having grid, filament and plate electrodes, an output circuit including a plate electrode, telephone receivers and a source of potential and means surrounding the conductors of said circuit and eziteiulinc substantially throughout the lengththereof and connected to said source of potential for shielding; said telephone receivers from extraneous electrical influences and regeneu ative efl'ects.

13. In a radio receiving apparatus, an electron tube amplifier comprising a plurality of electron tubes having grid, filament and plate electrodes, an input circuit, an output circuit, a responsive device connected in said output circuit, condlmtors for electrically connecting said device and said output circuit, 'and means for shielding said conductors from extraneous electrical influences and regenerative action with said input circuit comprising "a flexible metallic shield surrounding the conductors and exte'ndEng substantially throughout their length, and an GlBCtl'ltll connection between the shield, the responsive device, and one of said electrodes.

JULIUS WEINBERGER. 

